Add support for WLR_SESSION env variable

Valid values are "logind"/"systemd" and "direct". If WLR_SESSION is set,
only its value is potentially tried; it will not try any other option.

backend/session/session.c

@@ -66,11 +66,26 @@ static void handle_display_destroy(struct wl_listener *listener, void *data) {
 
 struct wlr_session *wlr_session_create(struct wl_display *disp) {
 	struct wlr_session *session = NULL;
-	const struct session_impl **iter;
 
+	const char *env_wlr_session = getenv("WLR_SESSION");
+	if (env_wlr_session) {
+		if (!strcmp(env_wlr_session, "logind") || !strcmp(env_wlr_session, "systemd")) {
+		#if defined(WLR_HAS_SYSTEMD) || defined(WLR_HAS_ELOGIND)
+			session = session_logind.create(disp);
+		#else
+			wlr_log(WLR_ERROR, "wlroots is not compiled with logind support");
+		#endif
+		} else if (!strcmp(env_wlr_session, "direct")) {
+			session = session_direct.create(disp);
+		} else {
+			wlr_log(WLR_ERROR, "WLR_SESSION has an invalid value: %s", env_wlr_session);
+		}
+	} else {
+		const struct session_impl **iter;
 		for (iter = impls; !session && *iter; ++iter) {
 			session = (*iter)->create(disp);
 		}
+	}
 
 	if (!session) {
 		wlr_log(WLR_ERROR, "Failed to load session backend");

docs/env_vars.md

@@ -16,6 +16,8 @@ wlroots specific
   of outputs
 * *WLR_NO_HARDWARE_CURSORS*: set to 1 to use software cursors instead of
   hardware cursors
+* *WLR_SESSION*: specifies the wlr\_session to be used (available sessions:
+  logind/systemd, direct)
 
 rootston specific
 ------------------